The invention relates to the preparation of liquid samples for analysis. In one aspect, the invention relates to a method for diluting the samples while in another aspect, the invention relates to an apparatus in which the samples are diluted. In yet another aspect, the invention relates to the sampling of a chemical-mechanical polishing slurry for the purpose of monitoring one or more properties of the slurry.
Liquids are used in many processes and applications. Often the composition of the liquid is important to the efficacy of the process or application, and often the composition of the liquid will change with use and/or time. Accordingly, such liquids are often monitored to ensure that their compositions remain within prescribed specifications.
One common form of monitoring a liquid used in a process involves obtaining a sample of the liquid and performing an off-line analysis upon it. Depending upon the process, the monitoring may require obtaining a plurality of samples either at one time or over a period of time. Depending upon the nature of the liquid and the analysis, and perhaps other factors as well, the liquid sample may or may not require some form of preparation before analysis. One form of preparation is dilution of the liquid sample.
By way of an example of a liquid that is used in a process and that requires frequent monitoring of its composition, chemical-mechanical polishing (CMP) systems are often employed in the microelectronics industry to planarize and/or polish semiconductor wafers. These systems typically contain and employ a “slurry” which is circulated throughout the system such that the slurry contacts and/or impinges upon the wafers. As the slurry impacts and/or passes over the wafers, the wafers are planarized and polished. One example of a slurry typically used in CMP systems is Semi-Sperse® 12 (SS-12) manufactured by Cabot Corporation of Aurora, Ill.
In order to maintain the consistency, performance, efficiency, and/or usefulness of the system, the “health” of the slurry must be maintained. Slurry instability, external contamination and/or process conditions (e.g., shear-inducing pressure gradients, flow rates, and exposure to air) may compromise slurry health. Thus, slurry properties (e.g., specific gravity, pH, weight percent solids, ionic contamination level, zeta potential, particle size distribution, etc.) are often closely monitored by a sampling system.
One such system for monitoring a CMP slurry is the intelligent Slurry Particle Equipment (iSPEQ) system which is described in commonly-owned, co-pending U.S. Ser. No. 60/313,440 filed Aug. 17, 2001 and entitled “Sampling and Measurement System with Multiple Slurry Chemical Manifold”. The iSPEQ typically comprises an AccuSizer 780/OL (manufactured by Particle Sizing Systems of Santa Barbara, Calif.), a multi-port valve manifold, a sample station, one or more system drains and an aspirator. The iSPEQ system uses a unique method for flushing its multi-port manifold. This method is described in commonly-owned, co-pending U.S. Ser. No. 60/313,439 filed Aug. 17, 2001 and entitled “Flushing a Multi-Port Valve Manifold”. In addition, the iSPEQ uses another method for collecting slurry from the sample station using the aspirator and multi-port manifold. This method is described in commonly-owned, co-pending U.S. Ser. No. 60/313,442 filed Aug. 17, 2001 and entitled “Chemical-Mechanical Polishing Sampling System Having Aspirator Drawn Pneumatics.”
The iSPEQ was primarily designed to measure the particle size distribution (PSD) of CMP slurries. Of all the slurry health parameters, perhaps the most important and frequently measured is the PSD of the bulk or “working” particles and the “large particle tail” of the PSD. Particle size distribution may be graphically represented by the concentration of particles as a function of particle diameter. In slurries such as SS-12, the PSD of the working particles is sufficiently approximated by a Gaussian Distribution where the peak or highest concentration of particles is centered between particles that are roughly 0.05 to 0.5 μm in diameter. Slurries typically contain extremely high concentrations of particles less than 0.5 μm in diameter. The region of the PSD that illustrates the concentration of particles for diameters greater than 0.5 μm is commonly referred to as the “large particle tail” of the PSD.
In the industry, the large particle tail can be measured using a variety of techniques (e.g. light scattering, light extinction, etc.) and instruments such as sensors, analyzers, and like devices (collectively referred to as sensors), that are commercially available from a host of manufacturers. Many different measurement devices have been tested, compared, and evaluated for their ability to measure PSDs, and optical particle counting is widely accepted as the most sensitive type of measurement technique.
In general, sensors based upon optical particle counting (these sensors are referred to as “optical particle counters” or “OPCs”) are used to measure the large particle tail of a slurry (those particles larger than about 0.5 μm in diameter). Optical particle counters count individual particles within a diluted slurry (e.g., silica, contaminants, debris, impurities, and the like) as the particles pass through a light beam. The slurry must be diluted enough so that only one detectable particle passes through the light beam at a time.
To produce a “diluted slurry” or “diluted liquid sample”, a diluent and a slurry are blended and/or mixed together. The diluent can include various grades of water, e.g., deionized, demineralized, ultra-pure, etc., as well as other liquids, e.g., water with a pH adjusted to that of the slurry sample, an organic solvent, etc. Depending upon a variety of factors, e.g., the nature of the slurry, the capabilities of the sensor, etc., proper dilution of the slurry sample for analysis may require several steps, i.e., the slurry sample is diluted to create a first diluted slurry, and then the first diluted slurry is itself diluted to create a second or further diluted slurry. This process can repeat itself as many times as necessary so as to achieve the desired diluted slurry for analysis.
To repeatedly generate a diluted slurry having an optimum “dilution ratio” (i.e., the ratio of the diluent volume to the slurry sample volume), some OPCs are integrated into an automatic dilution system. An example of a device that contains both a sensor and an “auto-dilution” system is the AccuSizer 780/OL (AccuSizer system). The AccuSizer system, as well as its auto-dilution apparatus, are described in detail in U.S. Pat. No. 4,794,806 (Nicoli, et. al.) and U.S. Pat. No. 5,835,211 (Wells, et. al.). Throughout this disclosure “AccuSizer system” refers to the combination of the auto-dilution apparatus and the sensor, and “AccuSizer sensor” refers to just the sensor component in the AccuSizer system.
The auto-dilution apparatus of the AccuSizer system and its operation are illustrated in and by FIGS. 1A and 1B. FIG. 1A is a table describing the ten operational steps of the AccuSizer system and the table contains typical times for each step, though the steps are not limited to these times. FIG. 1B is a schematic drawing of the system. The auto-dilution apparatus of the AccuSizer system is available in two formats, single dilution and double dilution. In the single dilution format, a slurry sample is captured in a sample loop, diluted in a mixer, and then the diluted sample is fed to a sensor for analysis. In the double dilution format, a slurry sample is captured in a sample loop, fed to a dilution chamber in which it is mixed with diluent to make a first diluted slurry, the first diluted slurry is then fed to the mixer in which it is mixed with additional diluent to make a second diluted slurry, and then this second slurry is fed to the sensor for analysis. The single dilution format is illustrated in FIG. 1B, and the double dilution format is also illustrated in FIG. 1B but with reference to the inset. The following description of the operation of the AccuSizer system is with respect to the double dilution format, yet the operation of the single dilution format is nearly the same but without reference to the dilution vessel (i.e., chamber).
The auto-dilution apparatus of the AccuSizer system operates in two stages. In the first stage (Steps 1-5), slurry sample is captured in a sample loop and diluted, and the sensor is prepared for slurry analysis. In the second stage (Steps 6-10), the slurry is analyzed and then flushed from the sensor.
The first stage starts with Step 1 of FIG. 1A, i.e., the simultaneous drawing of a fixed volume of sample (i.e., slurry) into the system and the flushing of dilution chamber 1 of FIG. 1B (i.e., the “vessel” in FIG. 1A). The function of Step 1 is two-fold, i.e., to capture sample for analysis and to ready the vessel to receive the sample.
During the sample loading sub-step of Step 1, valve SV15 is activated (i.e., opened) to capture a predetermined volume of slurry from slurry port 2, and syringe pump 3 is off. During the flushing of the dilution chamber (i.e., vessel flushing sub-step of Step 1), valves SV11 and SV12 are first opened, then valve SV11 is closed and valve SV14 is opened. Mass flow controller 4 is operational during the course of Step 1. Each sub-step of Step 1 takes about 30 seconds to complete but since these sub-steps occur simultaneously, the whole of Step 1 takes only about 30 seconds to complete.
In Step 2, mass flow controller 4 transfers deionized water (DI) into the system from DI port 5, through dilution chamber 1 and static mixer 6, and into sensor 7. During this operation, first valves SV11 and SV14 are opened, then SV12 and SV13 are opened; syringe pump 3 is initialized, and the mass flow controller is operational. Sensor 7 measures the background of the deionized water, which serves as the diluent for the slurry. Step 2 takes about 25 seconds to complete.
In Steps 3, 4 and 5, the slurry is diluted with the deionized and the diluted slurry is transferred to the sensor for analysis. The valve, syringe pump and the mass flow controller operations for these steps are described in FIG. 1A, and the time for each step is about 10, 40 and 0.5 seconds, respectively.
The second stage of the operation of the AccuSizer system's auto-dilution apparatus commences with Step 6, the actual analysis (i.e., the “measuring” of FIG. 1A) of the diluted slurry. This step takes about 60 seconds to complete and then in Steps 7-10, the diluted slurry is flushed from the system through exit port 8. The flush operations of Steps 8 and 9 are relatively long, e.g., about 90 seconds each, due to the need to insure that the sensor is rinsed clean of any residual slurry before the loading of another slurry sample into the system. The background check of Step 10 usually takes about 25 seconds to complete.
While the AccuSizer system's auto-dilution apparatus and others like it perform the basic task of diluting a slurry sample prior to its analysis by an OPC (or other sensor), it does so in a relatively inefficient manner. Each cycle of the AccuSizer system takes approximately 6 or more minutes to complete, but during this time data, e.g., PSD analysis of the diluted sample, is only collected for 60 seconds. The bottlenecks in this system are many, and they include sample capture, flush steps, background checks and the transfer of the sample to the dilution chamber by a syringe pump. Accordingly, the industry has a continuing interest in a dilution system that allows for more slurry analysis in less time with the concurrent elimination of one or more of the bottlenecks of the present systems. More generally, all industries have a continuing interest in performing efficient monitoring of the liquids used in their processes, and the elimination or moderation of any bottlenecks in these monitoring processes is always welcomed.